This invention relates to miniature electric motors and in particular, to the housing for a miniature motor.
Miniature motors vary in size and with small miniature motors, the material of the rear housing part is relatively thin, allowing an end cap or bearing plate to be secured to the rear housing by crimping the rear housing. This crimping process may involve bending over an axially extending finger cut from the edge of the rear housing or simply deforming discrete parts of the edge of the rear housing.
As the size and power of the motor increases, it is also desirable to increase the thickness of the rear housing as it forms the magnetic flux return path for the stator.
However, as the thickness of the rear housing increases so does the power required to crimp the end plates to the metal housing. The metal also exhibits resilience so that when the cut finger crimping method is used with a thick wall housing, say in the order of 2 mm, the finger springs back slightly. While the end cap is still captured by the finger, it is not held firmly against the metal housing resulting in play or movement between the metal housing and the end cap.
Another method used is to cut holes in the housing so as to form pairs of opposing circumferentially extending fingers. Sometimes the fingers may be joined together. The end plate is crimped to the housing by radially deforming the fingers over a crimping surface of the end cap thereby preventing axial separation of the end cap and the housing.
However, while the connection is not affected by spring back of the metal fingers, the degree of tightness of the connection will depend on the relative dimensions of the end cap and housing crimping portions. For this reason, such connections are used mostly with metal end plates. If the end plate thickness is too small or the fingers are too narrow then there will be a gap between the end plate and the finger giving a loose connection between the end plate and the housing. If the end plate is too thick or high and/or the fingers are too broad, then the crimping will not be successful, excess force may be applied damaging the motor or the fingers may be deformed leading to possible damage or mounting and handling problems, as well as possible clamping relaxation problems.
Thus there is a need for a method of connecting an end cap to a motor housing which will securely fix the end cap to the housing in a simple yet quick and effective manner.
This need is fulfilled by this invention by using locking tabs in the form of circumferentially extending fingers which are moved radially to clamp axially the end cap to the housing.
Accordingly, in one aspect thereof, the present invention provides a miniature electric motor comprising: a tubular housing; and at least one end cap, said end cap having a boss portion fitted inside the housing, a flange which abuts an axial end of the housing, and an engagement portion; said housing having at least one circumferentially extending finger which is radially deformed into contact with the engagement portion of the end cap to capture the end cap to the housing; wherein the engagement portion includes an axially projecting ridge and the finger bears axially onto the ridge to nip the end cap to the housing.
According to a second aspect thereof, the present invention provides a method of connecting an end cap to a tubular housing of a miniature electric motor, the method comprising the steps of providing an end cap with a flange portion, a boss portion and at least one engagement portion, providing a tubular housing with at least one circumferentially extending finger, inserting the boss portion of the end cap into the housing such that the flange abuts an axial end of the housing and the recess is aligned with the finger, and providing an axially projecting ridge on the engagement portion and deforming the finger radially so that an axially inner edge of the finger axially engages the ridge to prevent axial movement of the end cap with respect to the housing.
Preferably, the method also includes providing two pairs of said fingers and radially deforming each pair of fingers into a respective engagement portion of the end cap, each finger having an axially inner edge which extends at an incline to a plane orthogonal to an axis of the housing, the axially inner edge being brought into contact with an axially inner surface of the recess by radially deforming and continuing to radially deform the finger causing the inner edge of the finger to exert an axial force on the surface of the end cap to clamp the end cap to the housing.